The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to MRI imaging systems equipped with a magnet assembly positioned under a table or at an end of the table.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is the x-y plane and which is near the Larmor frequency, the net aligned moment, M2, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment M. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In a typical magnetic resonant imaging (MRI) system, the magnets utilized have a donut shape structure. The opening or bore in the magnet structure receives the patient and patient table during the MRI scan. In some cases, either because of characteristics of the patient, such as the patient""s size, or because of instruments and equipment that are attached to the patient, it is difficult or uncomfortable for the patient to be inserted into the bore of the magnets. Additionally, in some cases, a claustrophobic effect is experienced by the patient and result in difficulties in obtaining the MRI scan.
Thus there is a need for an MRI apparatus that provides a magnet assembly positioned either alongside the table or underneath the table that efficiently produces a sufficiently homogeneous magnetic field for imaging. There is also a need for MRI apparatus that uses an non-homogeneous pre-polarizing magnet in order to increase the signal to noise ratio associated with a homogeneous imaging magnet.
The present invention provides a method for obtaining an image of a selected portion of a subject of interest with a magnetic resonant imaging apparatus. The method comprises the steps of positioning a subject of interest within the magnetic field geometry of a magnet assembly, which includes an imaging magnet and energizing, rapidly, a polarizing electromagnet. The polarizing electromagnet is maintained in the energized mode for a pre-determined period of time and then rapidly de-energizing the polarizing electromagnet. Then an RF coil and gradient coil are energized to acquire the image. The methods can also include the steps of providing a table and positioning the magnet assembly below the table or positioning the magnet assembly, at one end of the table and a side of the table in a perpendicular aspect with respect to the table. A partition member can also be disposed between the patient table and the magnet assembly. The table can be a patient table.
The present invention also provides a magnetic resonant imaging apparatus having an operator console with input and output devices, a computer system for acquiring, processing and storing MRI images, and a system control device for generating and acquiring MRI image data. The magnetic resonant imaging apparatus comprises a patient table with a first electromagnet producing a polarizing magnetic field in a spaced apart relationship from the table, a second electromagnet producing an imaging magnetic field in a spaced apart relationship from the table and the first electromagnet, and magnetic gradient coils and RF coils disposed between the table and the first and second electromagnets and within geometry of the magnetic fields produced by the electromagnet. An examplary embodiment of the magnetic resonant imaging apparatus provides that the magnetic gradient coil for the x-axis and the magnetic gradient coil for the y-axis are energized. It also provides that the RF coil transmits and receives an RF pulse to obtain a magnetic resonant image of a subject of interest (SOI). Another embodiment of the magnetic resonant imaging system provides that the first and second electromagnets are solenoidal.
The present invention further provides a magnetic resonant imaging apparatus having an operator console with input and output devices, a computer system for acquiring, processing and storing MRI images, and a system control device for generating and acquiring MRI image data, with the magnetic resonant imaging apparatus comprising a means for supporting a subject of interest, a means for producing a polarized magnetic field in a spaced apart relationship from the means for supporting, a means for generating a magnetic field to produce an image of a subject of interest, with the means for generating in a spaced apart relationship from the means for supporting and the means for producing, and a means for emitting gradient magnetic fields and means for transmitting and receiving RF signals disposed between the means for supporting and the means for producing and the means for generating and within the magnetic fields produced by the means for producing and the means for generating.